1. Field of the Invention
The present invention generally relates to the operation of a coordinate measuring machine (CMM) and the like. More particularly, the present invention relates to a technique for determining tolerance measurement values and estimating nominal measurement values for the CMM operator to reduce data entry and operator error.
2. Description of the Prior Art
Coordinate measuring machines (CMM's) have been used for years for industrial quality control to inspect the products of computer numerical control (CNC) machine tools and to check feature locations of parts after machining. Although the CMM was once considered an exotic tool found only in large manufacturing plants, the increased need for accurate measurement of components has become an important concern for small job shops as well. Accordingly, modern CMMs are significantly smaller and more affordable, and are becoming commonplace in the average machine shop.
However, in the past, the calibration and operation of a coordinate measuring machine was a slow and tedious process, typically designed to be used by skilled quality control inspectors. Even today, most CMMs are designed to be operated by a single, skilled user, typically for measuring either a single type of part or a series of the same type of parts. The CMM operator often uses a predetermined or "canned" part program in the "Run Mode" of operation to check tolerances on a production run of identical parts. More sophisticated CMM operators can create the part programs to perform simple measurements and provide basic inspection reports. That is not the case, however, for the machinist in the small job shop who only occasionally needs to use a CMM. In many of the smaller shops, these "walk-up users" are trained CNC machine tool operators, toolmakers, etc., who only need to make a few quick measurements to verify a tool setup. The walk-up user does not have extensive training in the operation of the CMM, and may have little or no familiarity with the setup or calibration procedure of the machine. Hence, the walk-up user requires a simple, fast, user-friendly computer interface that will allow him to quickly make his measurements and get back to machining the part.
One problem associated with the CMM for the small job shop is the time and effort it takes to have the operator input information regarding measurement tolerance values and nominal values. The nominal value represents the measurement value to which the part was optimally designed to meet, while the tolerance value represents the deviation amount, either high or low of the nominal, that will be acceptable. In other words, each machined part has a nominal value and a tolerance value or range which defines the acceptable range for a part.
These nominal values and tolerance values are specified on the mechanical drawing for every feature of the part to be measured. For example, when the operator measures the diameter of a circle and obtains a result of 1.997 inches, he must go to the report screen or setup screen and enter the nominal value and tolerance values from the blueprint. The circle may have a nominal diameter of 2.000 inches with a tolerance of 0.005 inches. If the user does not enter these numbers into the computer, they will not appear on the results page of the report, and thus, the user will not have a useful record of whether the part is acceptable or not. Unless the operator individually inputs each nominal and tolerance value before continuing with the measurement, most CMMs will default to zero such that all measurements will appear to be out of specification.
The main problem lies in the realization that there may be one hundred or more part features to measure for a particular part, and several characteristics to be measured for each part feature. This would result in the operator having to key-in hundreds of measurement values for each report. Not only is this data entry extremely tedious and time-consuming, but each time the operator manually enters data into the CMM system, there is provided one more opportunity for a measurement error to occur, due either to incorrect data entry or the misreading of a blueprint.
A need, therefore, exists for a technique for automating this process such that the CMM operator does not have to spend time entering nominal measurement values and their tolerances for each part feature characteristic to be measured.